Add specs for Hyrax::Group

Also fixes an incorrect implementation of `Hyrax::Group.from_key`.

The Sipity specs were inspired by the similar specs for `Hyrax::User`.

app/models/hyrax/group.rb

@@ -10,7 +10,7 @@ def self.name_prefix
     ##
     # @return [Hyrax::Group]
     def self.from_key(key)
-      new(key.slice!(name_prefix))
+      new(key.delete_prefix(name_prefix))
     end
 
     def initialize(name)

spec/models/hyrax/group_spec.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+RSpec.describe Hyrax::Group, type: :model do
+  let(:name) { 'etaoin' }
+  let(:group) { described_class.new(name) }
+
+  describe '.from_key' do
+    it 'returns an equivalent group' do
+      expect(Hyrax::Group.from_key(Hyrax::Group.name_prefix + group.name)).to eq group
+    end
+  end
+
+  describe '#==' do
+    let (:other_group) { described_class.new(group.name) }
+
+    it 'correctly determines equality for equivalent groups' do
+      expect(other_group).to eq group
+    end
+  end
+
+  describe '#name' do
+    it 'returns the name' do
+      expect(group.name).to eq name
+    end
+  end
+
+  describe '#to_sipity_agent' do
+    subject { group.to_sipity_agent }
+
+    it 'will find or create a Sipity::Agent' do
+      expect { subject }.to change { Sipity::Agent.count }.by(1)
+    end
+
+    context "when another process makes the agent" do
+      before do
+        group.to_sipity_agent # create the agent ahead of time
+      end
+
+      it "returns the existing agent" do
+        expect { subject }.not_to change { Sipity::Agent.count }
+      end
+    end
+  end
+end